A method that can enhance the efficiency of CELES a lot is described in [this paper by Chew](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwjyz8SEv9faAhUoxKYKHYa0BOkQFggxMAE&url=https%3A%2F%2Fwww.researchgate.net%2Fpublication%2F2738583_An_FFT_T-Matrix_Method_for_3D_Electromagnetic_Scattering_Solutions_from_Random_Discrete_Scatterers&usg=AOvVaw0Pr4dRFbFCYlBoyB2_23Bu). Implementing this would be a bigger project, but should be doable.